Internal combustion engines typically use either a mechanical, electrical, or hydro-mechanical valve actuation system to actuate the engine valves. These systems may include a combination of camshafts, rocker arms and pushrods that are driven by the engine's crankshaft rotation. When a camshaft is used to actuate the engine valves, the timing of the valve actuation may be fixed by the size and location of the lobes (i.e., cams) on the camshaft.
For each 360 degree rotation of the camshaft, the engine completes a full cycle made up of four strokes (i.e., expansion, exhaust, intake, and compression). Both the intake and exhaust valves may be closed, and remain closed, during most of the expansion stroke wherein the piston is traveling away from the cylinder head (i.e., the volume between the cylinder head and the piston head is increasing). During positive power operation, fuel is burned during the expansion stroke and positive power is delivered by the engine. The expansion stroke ends at the bottom dead center point, at which time the piston reverses direction and the exhaust valve may be opened for a main exhaust event. A lobe on the camshaft may be synchronized to open the exhaust valve for the main exhaust event as the piston travels upward and forces combustion gases out of the cylinder.
Additional auxiliary valve events, while not required, may be desirable and are known to provide alternative flow control of gas through an internal combustion engine in order to, for example, provide vehicle engine braking. For example, it may be desirable to actuate the exhaust valves for compression-release (CR) engine braking, bleeder engine braking, exhaust gas recirculation (EGR), brake gas recirculation (BGR), or other auxiliary valve events. Furthermore, other positive power valve motions, generally classified as variable valve actuation (VVA) event, such as but not limited to, early intake valve opening (EIVC), late intake valve closing (LIVC), early exhaust valve opening (EEVO) may also be desirable. Further still, cylinder deactivation (or variable displacement), in which engine valves remain closed and fuel is not provided to a given cylinder thereby effectively removing that cylinder from positive power production, may be desirable to improve engine operating efficiency under comparatively low load conditions.
One method of adjusting valve timing and lift given a fixed cam profile has been to incorporate a lost motion device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion dictated by a fixed cam profile with a variable length mechanical, hydraulic or other linkage assembly. In a lost motion system a cam lobe may provide the maximum dwell (time) and greatest lift motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage intermediate of the valve to be opened and the cam providing the maximum motion to subtract or “lose” part or all of the motion imparted by the cam to the valve. This variable length system, or lost motion system may, when expanded fully, transmit all of the cam motion to the valve and when contracted fully transmit none or a minimum amount of the cam motion to the valve.
Such known conventional systems may not provide the desired level of engine braking power, particularly in the case of downsized engines and/or heavier loads requiring more braking power than currently available with conventional compression release engine braking. It is known that engine braking valve motion with a second compression release event (i.e., 2-stroke engine braking) can provide the necessary braking power from the engine brake. Unfortunately, however, many engines do not have sufficient room to include the necessary components to effect the various above-noted auxiliary valve events, particularly those related to 2-stroke engine braking. To overcome such space issues, it is possible to incorporate such components into relatively large (and consequently expensive) overhead housings.
Thus, it would be advantageous to provide solutions for engine braking and other auxiliary valve movement regimes that overcome the limitations of conventional systems.